PROJECT SUMMARY?ABSTRACT Multiple myeloma (MM) is a genetically heterogeneous disease, with varying levels of genomic instability and intra-tumor clonal heterogeneity. Both levels of heterogeneity pose challenges for directed anti-tumor targeted therapy, which may be overcome by harnessing the power of the immune system. LCL161 is a SMAC mimetic targeting the cellular inhibitor of apoptosis proteins cIAP1 and -2 (cIAP1/2). We previously reported that bi- allelic deletions of cIAP1/2 in MM result in activation of the non canonical NFkB pathway, and consistently LCL161 has little direct anti-tumor activity, against primary MM cells, MM cell lines, or xenograft models where it induces even higher levels of NFkB. However, LCL161 has dramatic activity in vivo against MM that develops spontaneously in the immuno-competent and clinically predictive Vk*MYC mouse model, but not in vitro against these same tumor cells, suggesting an important role for the host in mediating the anti-tumor effect. Transplantation and cell depletion experiments in Vk*MYC mice implicated macrophages as mediator of LCL161 anti-MM activity that does not depend on adaptive immunity. M? are a crucial component of the MM niche and support MM growth by providing survival signals and promoting immunosuppression. However macrophages can be re-educated to acquire tumoricidal activity by CSF1R inhibition, anti-CD47 treatment or combinations of Toll-like-receptor (TLR) agonists + IFNg, TLRa + CD40 agonist or chemotherapy. We found that the SMAC mimetic compound (SMC) LCL161 also promotes macrophages M1 polarization and potent anti-MM activity in vivo. Specifically, LCL161 treatment induced M1 polarization and tumoricidal activity in BM derived macrophages co-cultures experiments, and induction of M1 cytokines in plasma from MM patients enrolled in the clinical trial. The goal of this proposal is to define the mechanisms implicated in LCL161 anti-MM activity in Vk*MYC mice and in samples obtained from MM patients enrolled in a phase II clinical trial of LCL161 at the Mayo Clinic.